Physical Properties
Alcohols have dramatically higher boiling points and much greater water solubility than alkanes of comparable molecular weight. Both facts trace back to a single feature: the O-H group can form hydrogen bonds with its neighbors. Every physical property of alcohols - boiling point, density, solubility, viscosity - follows from this one detail.
Understanding alcohol physical properties also gives you a template for every polar functional group you will meet later. Amines, carboxylic acids, and amides all participate in hydrogen bonding, and their properties scale with the number and strength of H-bonds they form.
Hydrogen Bonding in Alcohols
A hydrogen bond is an electrostatic attraction between a hydrogen atom bonded to a highly electronegative atom (O, N, or F) and a lone pair on a nearby electronegative atom. Each alcohol molecule is both a hydrogen-bond donor (O-H) and an acceptor (lone pairs on O). In pure alcohol, every molecule is hydrogen-bonded to several neighbors at once, forming a dynamic network.
Hydrogen bonds are strong for an intermolecular force (about 5-30 kJ/mol per bond), though they are roughly a tenth the energy of a typical covalent bond. To boil the liquid, enough hydrogen bonds have to break to let molecules escape into the gas phase - which requires a lot of energy.
Boiling Points: Alcohols Beat Alkanes and Ethers
Compare the boiling points of molecules with similar molecular weight:
| Compound | MW | Boiling point | IMF |
|---|---|---|---|
| Butane (C₄H₁₀) | 58 | −0.5°C | London only |
| Diethyl ether (C₂H₅OC₂H₅) | 74 | 35°C | Dipole-dipole + London |
| 1-Butanol (C₄H₉OH) | 74 | 118°C | H-bond + dipole + London |
| Water (H₂O) | 18 | 100°C | H-bond (very strong) |
Butane has only London forces. Diethyl ether adds dipole-dipole (the C-O bonds are polar) - boiling point jumps by ~35°C. Butanol adds hydrogen bonding - boiling point leaps by another ~80°C. The OH group is the biggest single factor in raising boiling point per unit mass.
Boiling Point Trends Within Alcohols
Three structural features drive alcohol boiling point:
- Chain length (MW): longer chain = more London forces = higher boiling point. Ethanol boils at 78°C; hexanol boils at 158°C.
- Branching: branched alcohols have lower surface area contact and weaker London forces, so they boil lower than straight-chain isomers. n-Butanol (118°C) beats tert-butanol (82°C).
- Number of OH groups: each additional OH adds more hydrogen-bonding capacity. Ethylene glycol (197°C) boils much higher than 1-butanol of similar MW, and glycerol (290°C) higher still.
Water Solubility: The Tail Wags the Dog
Short-chain alcohols (methanol, ethanol, propanol) are miscible with water in all proportions. Longer-chain alcohols become less water-soluble as the hydrophobic carbon chain outweighs the hydrophilic OH:
| Alcohol | Water solubility |
|---|---|
| Methanol (C1) | Miscible (infinite) |
| Ethanol (C2) | Miscible |
| 1-Propanol (C3) | Miscible |
| 1-Butanol (C4) | 80 g/L |
| 1-Pentanol (C5) | 25 g/L |
| 1-Hexanol (C6) | 6 g/L |
| 1-Octanol (C8) | 0.5 g/L |
The rule of thumb: one OH group can solvate about 3-4 carbons of hydrophobic tail. Beyond that, the molecule starts to behave more like an alkane and less like an alcohol.
This trade-off - polar head vs. hydrophobic tail - is the exact same logic behind lipid structure. A long-chain fatty acid is essentially a fatty alcohol with a carboxylic acid head - the fatty acid’s polar end dissolves in water while the long hydrocarbon tail avoids it. That is why cell membranes form lipid bilayers.
Density
Short-chain alcohols are less dense than water (ethanol density ≈ 0.79 g/mL), so pure ethanol floats. Longer-chain alcohols approach 0.83 g/mL. Methanol, ethanol, and isopropanol all form azeotropes with water (constant-boiling mixtures that cannot be separated by simple distillation), which has practical consequences for purification.
Why Ethers Boil Lower Than Alcohols
Ethers (R-O-R’) have the same polar C-O bonds but no O-H bond. They can accept hydrogen bonds (via the oxygen’s lone pairs) but cannot donate them. The result: weaker aggregate intermolecular forces, lower boiling points, and lower solubility in water than similar-MW alcohols.
Diethyl ether (MW 74, b.p. 35°C) vs. 1-butanol (MW 74, b.p. 118°C) illustrates this gap. Ethers are still slightly soluble in water because they can accept H-bonds from water, but they are much less hygroscopic than alcohols.
Acidity Preview
Alcohols are weakly acidic, with pKa typically 16-18 (ethanol ≈ 16, methanol ≈ 15.5). The full acidity story is the subject of Section 5.3. For now, know that the O-H bond is the most acidic position in the molecule, and the conjugate base (alkoxide, RO⁻) is the reactive species in many subsequent reactions.